1. Field of the Invention
The present invention relates to a light power control circuit for controlling a light power of a semiconductor light-emitting element for use in an information recording and reproducing system employing an optical disk.
2. Description of Background Information
In optical information recording/reproducing systems for disks such as direct readable (DRAW (optical direct read after write)) disks or erasable direct readable (EDRAW (erasable direct read after write)) disks, a laser diode, that is a semiconductor light-emitting element, is used as a light source of a light beam for writing data on the optical disk or reading the written data from the optical disk. In light power control circuits for a laser diode, a high frequency current having a frequency above 100 MHz is superposed on the drive current of the laser diode in order to prevent degradation of the C/N ratio of the read signal as a result of a change in the oscillation mode of the laser diode caused by a reflected light returning from the disk surface. With such a provision, the oscillation of the laser diode becomes stable against the reflected light (see Japanese Patent Publication No. 59-9086, for example).
Conventionally, the superposition of the high frequency current is performed of course in the data reading mode, and also in the data writing mode in which the laser beam is emitted at high power unless the maximum rating of the laser diode is exceeded by the superposition of the high frequency current. However, the high frequency current for the superposition is not synchronized with writing data, a cross modulation distortion is generated by the interference between the data signal and the high frequency superposing signal if the superposition is performed in the writing mode. Consequently, the writing data is recorded with its edges being periodically shifted in the time axis, causing degradation of the recording signal.
An example of conventional light power control circuit is shown in FIG. 1. As shown, a semiconductor light-emitting element, i.e., a laser diode 1 is used as a light source for generating a writing/reading light beam for writing data and reading written data on and from a recording medium. A monitor diode 2 operating as a light sensing device is incorporated in an optical head together with the laser diode 1. The monitor diode 2 receives a part of the light emitted by the laser diode 1, and its output signal is supplied to a sample and hold circuit 4' through a monitor amplifier 3. The sample and hold circuit 4' consists of a switch SW.sub.3, connected in series with an output terminal of the monitor amplifier 3, and a capacitor C.sub.1 inserted between an output terminal of the switch SW.sub.3, and ground. The switch SW.sub.3, turns off (opens) during a writing mode and turns on (closes) during a reading mode, in response to a writing gate pulse which is generated during the period of each section of writing data and is supplied from a controller (not illustrated).
The output signal of the sample and hold circuit 4' is supplied to an inverting input terminal of an operational amplifier OP which constitutes an integrator 6 together with a capacitor C. A voltage corresponding to a reading power set value determined by the reading power setting circuit 7 is applied to a non-inverting input terminal of the operational amplifier OP. By this structure, a voltage corresponding to the difference between the output voltage of the sample and hold circuit 4' and the voltage corresponding to the reading power set value is derived. The output signal of the integrator 6 is converted to a current by means of a V-I (voltage to current) converter 8, and in turn supplied to a laser diode 1 through an adder 9 as a writing drive current.
For writing data, a greater laser power than that in the reading time is needed, and the laser power value for the writing operation is set by a writing power set circuit 10. The voltage corresponding to the set value of the writing power, which is issued from the writing power set circuit 10, is converted to a current at another V-I converter 11 and in turn supplied to a switch SW.sub.1. The switch SW.sub.1 is positioned on the ground side during the reading mode. During the writing mode, the switch SW.sub.1 is on/off operated in response to the writing data, so that the current corresponding to the writing power set value is modulated in response to the writing data. The output current of the switch SW.sub.1 is superposed on the reading drive current at the adder 9, and the obtained current is supplied to the laser diode 1 as a writing drive current.
The sample and hold type light power control circuit described above is constructed that a light power sensing voltage sensed by the monitor diode 2 in the reading time immediately before the writing mode is stored in the capacitor C.sub.1 at the data writing time, and the voltage corresponding to the writing power set value is added to the hold voltage. Therefore, no problem arises if the recording format is such that the laser power returns to the reading power at predetermined intervals, such as in the data recording. However, if the recording is performed for a long time, the light power sensing voltage at the reading time cannot be maintained because of discharge of the capacitor C.sub.1. In such a case, a problem arises that the mode of the operation cannot be changed immediately to the reading mode from the writing mode.
Moreover, it is also possible to use an A/D converter and a D/A converter instead of the sample and hold circuit 4'. However, even if A/D and D/A converters are used, current-light power characteristic curve will be changed if the writing time is prolonged, due to the heat generated by the laser diode 1. As a result, it is not possible to hold the light power sensing voltage sensed during the reading time, and the mode cannot be changed immediately from the writing mode to the reading mode also in this case.
Furthermore, there is a known control circuit disclosed in Japanese Patent provisional publication No. 59-146457 as another example of the light power control circuit. In the case of this control circuit an average value of the writing data is produced, and a stable operation of the system is attained by multiplying the average value to the writing power set value, then subtracting the obtained multiplied value from the light power sensing voltage generated by the light sensor. However, in such a control circuit, a problem of drift and the necessity of an off-set adjustment inevitably arise because of the use of a multiplier for multiplying the average value of writing data and the writing power set value.